Method for manufacturing high strength steel sheet

ABSTRACT

A method for manufacturing a high strength steel sheet includes heating a steel sheet containing at least 0.10 mass % of carbon to either a temperature in an austenite single phase region or a temperature in an (austenite+ferrite) two-phase region; cooling the steel sheet to a cooling stop temperature as a target temperature set within a cooling temperature region ranging from Ms to (Ms−150° C.) to allow a portion of non-transformed austenite to proceed to martensitic transformation; retaining a coldest part in a sheet widthwise direction of the steel sheet at a temperature in a temperature range from the cooling stop temperature as the target temperature to (the cooling stop temperature+15° C.) for 15 seconds to 100 seconds; and heating the sheet to a temperature to temper said martensite, wherein “Ms” represents martensitic transformation start temperature and said cooling temperature region is exclusive of Ms and inclusive of (Ms−150° C.).

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2011/001163, with an inter-national filing date of Feb. 28, 2011 (WO 2011/111332 A1, published Sep. 15, 2011), which is based on Japanese Patent Application No. 2010-052323, filed Mar. 9, 2010, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a method for manufacturing a high strength steel sheet being excellent in formability in particular ductility and stretch-flangeability and having tensile strength of at least 980 MPa for use in the industrial fields of automobiles, electric appliances and the like.

BACKGROUND

Improving fuel efficiency of automobiles has been an important task in recent years from the viewpoint of global environment protection. Due to this, there has been vigorous trend toward making vehicle body parts thin by increasing the strength of vehicle body materials to reduce weight of vehicles.

In general, the proportion of a hard phase such as martensite and bainite with respect to the entire microstructure of a steel sheet must be increased to increase the strength of the steel sheet. However, enhancing the strength of a steel sheet by increasing the proportion of a hard phase thereof tends to deteriorate formability of the steel sheet. Therefore, there has been a demand for a steel sheet having both high strength and good formability in a compatible manner. There have been developed up to now various types of multi-phase steel sheets such as ferrite-martensite dual phase steel (DP steel), TRIP steel utilizing transformation-induced plasticity of retained austenite, and the like.

In a case where the proportion of a hard phase is increased in a multi-phase steel sheet, formability of the steel sheet is strongly influenced by formability of the hard phase because not only deformability of polygonal ferrite, but deformability of a hard phase itself directly affects formability of the steel sheet. As a result, formability of a resulting steel sheet significantly deteriorates if formability of the hard phase is insufficient as described above where the proportion of a hard phase is increased. In contrast, deformability of polygonal ferrite dominates formability of a steel sheet to ensure good formability, e.g., good ductility, in spite of poor formability of a hard phase where the steel sheet contains soft polygonal ferrite at a relatively high content and the hard phase at a relatively low content.

In view of this, there have conventionally been attempts to: subject a cold rolled steel sheet to a thermal treatment to adjust the content of polygonal ferrite therein generated by our annealing process and cooling process thereafter; allow martensite to be formed by water-quenching the steel sheet thus treated; and temper martensite by heating the steel sheet to relatively high temperature and retaining the steel sheet in that state to allow carbides to form in martensite as a hard phase, thereby improving formability of martensite.

In such a case of employing such conventional facilities for continuous annealing and water-quenching as described above, however, the temperature of a steel sheet after quenching naturally drops to a temperature around the water temperature and most of non-transformed austenite experiences martensitic transformation, whereby it is difficult to utilize low-temperature transformed microstructure such as retained austenite and the like. In other words, improvement of formability of a hard microstructure totally depends on an effect caused by martensite tempering. Improvement of formability of a steel sheet is thus significantly limited in the case of employing facilities for continuous annealing and water-quenching.

Alternatively, there has been proposed as a steel sheet having a hard phase other than martensite a steel sheet including polygonal ferrite as a main phase and bainite and pearlite as hard phases with carbides formed in bainite and pearlite as the hard phases. This steel sheet aims to improve formability thereof not only by use of polygonal ferrite as the main phase, but also by formation of carbides in the hard phases to improve formability in particular stretch-flangeability of the hard phases themselves.

JP-A 04-235253, example, proposes a high tensile strength steel sheet having excellent bendability and impact properties, manufactured by specifying alloy components and obtaining steel microstructure constituted of fine and uniform bainite having retained austenite.

JP-A 2004-076114 proposes a multi-phase steel sheet having excellent bake hardenability, manufactured by specifying types and contents of alloy components, obtaining steel microstructure mainly constituted of bainite having retained austenite and controlling the content of the retained austenite in bainite.

Further, JP-A 11-256273 proposes a multi-phase steel sheet having excellent impact resistance, manufactured by specifying types and contents of alloy components, obtaining steel microstructure including at least 90% (by area ratio) bainite having retained austenite and 1%-15% retained austenite in bainite and setting hardness (HV) of bainite in a specific range.

The aforementioned steel sheet, however, has problems described below.

The component composition described in JP-A 04-235253 cannot ensure a sufficient content of stable retained austenite to express a TRIP effect in a high strain region of a resulting steel sheet when the steel sheet is imparted with strains, whereby the steel sheet exhibits poor ductility prior to reaching plastic instability and poor stretchability, although bendability thereof is relatively good.

The steel sheet of JP-A 2004-076114, although it has good bake hardenability, experiences difficulties not only in achieving high tensile strength (TS) equal to or higher than 980 MPa or 1050 MPa, but also in ensuring satisfactory formability such as ductility and stretch-flangeability when strength thereof is ensured or increased due to its microstructure primarily containing bainite or ferrite with martensite reduced as best as possible.

The steel sheet of JP-A 11-256273 primarily aims to improve impact resistance and the microstructure thereof includes as a main phase bainite having hardness (HV) of 250 or less at a content exceeding 90%, whereby it is very difficult to achieve tensile strength (TS) of at least 980 MPa in that steel sheet.

It is reasonably assumed that, among automobile parts to be formed by press-forming, automobile structural members having relatively complicated shapes such as a center pillar inner generally require a tensile strength of at least 980 MPa and, in the future, possibly at least 1180 MPa class.

Further, a steel sheet for use as a material of vehicle parts requiring high strength in particular such as a door impact beam, a bumper reinforcement to suppress deformation during a car collision generally necessitates a tensile strength of at least 1180 MPa class and, in the future, possibly at least 1470 MPa class.

Various types of steel sheets have been developed as described above as the demand for a steel sheet having higher strength increases. It is very important to ensure good stability in mechanical properties of a high strength steel sheet in terms of reliably obtaining good formability of the steel sheet in a stable manner. In view of this, there has been developed, for example, a multi-phase high strength steel sheet including various types of hard microstructures manufactured by utilizing various types of hard microstructures, transformed from non-transformed austenite in a relatively low temperature range to avoid having an overall microstructure constituted of a single phase such as martensite. It is very important in such a multi-phase microstructure as described above to control fractions of respective hard phases or microstructures with good precision in terms of stabilizing mechanical properties of the resulting multi-phase high strength steel sheet. However, precision in fraction control is not yet sufficiently high in such a case as described above.

Specifically, variation in sheet temperature within a steel sheet tends to occur when the steel sheet is subjected to thermal treatment such as finish annealing. Accordingly, when such a steel sheet having a variation in sheet temperature as described above is rapidly cooled to a target temperature to allow martensite to be formed by a predetermined content, martensite is not formed at a uniform content, but the formation ratio of martensite rather varies across the steel sheet due to the aforementioned variation in sheet temperature. As a result, there arises variations in mechanical properties of the resulting steel sheet.

It could therefore be helpful to provide a method for manufacturing a high strength steel sheet having tensile strength (TS) of at least 980 MPa, being excellent in formability in particular ductility and stretch-flangeability and exhibiting good stability in mechanical properties.

Specifically, it could be helpful to provide a high strength steel sheet having high strength and good formability in a compatible manner by transforming a portion of non-transformed austenite into tempered martensite and the rest of the non-transformed austenite into microstructures such as bainite and retained austenite. The high strength steel sheet should also include a steel sheet of which surface has been further treated by hot dip galvanizing or galvannealing.

SUMMARY

Hereinafter, “Being excellent in formability” represents a condition that a product of tensile strength and total elongation, i.e., (TS×T. EL), is equal to or higher than 20000 MPa·% and a condition that a product of tensile strength and critical hole expansion ration, i.e., (TS×λ), is equal to or higher than 25000 MPa·% are both satisfied. Further, “Being excellent in stability of mechanical properties” represents that the standard deviation σ of TS in the sheet widthwise direction and the standard deviation σ of T. EL are not larger than 10 MPa and not larger than 2.0%, respectively.

Wherein a desired microstructure, e.g., a predetermined ratio of martensite, is to be formed in a steel sheet, the steel sheet is cooled to particular target temperature which is set accordingly. However, the steel sheet to be thus cooled tends to have variation in sheet temperature due to the preceding thermal treatment as described above. Accordingly, in a case where a such a steel sheet having variation in sheet temperature thereof as described above is cooled and when the temperature of a part of the steel sheet where temperature is lowest (the coldest part) reaches the target temperature as shown in FIG. 1( a), martensite has not been so sufficiently formed in a part of the steel sheet where temperature is highest (the hottest part) as in the coldest part, whereby variation arises in microstructure of the steel sheet. Meanwhile, when the temperature of the hottest part of the steel sheet reaches the target temperature as shown in FIG. 1( b), martensitic transformation has proceeded too far in the coldest part of the steel sheet, thereby worsening variation in microstructure of the steel sheet.

In short, variation in sheet temperature within a steel sheet results in non-uniform microstructure of steel and thus inevitably in variation in mechanical properties of the steel sheet.

We discovered that the microstructure of a steel sheet is made uniform and thus variations in mechanical properties such as strength of the steel sheet can be reduced by setting thermal treatment conditions around a target temperature to select the coldest part of a steel sheet as the reference region, cool the coldest part to the target temperature as shown in FIG. 1( c), and retain the steel sheet in a temperature range slightly above target temperature for a predetermined time.

We thus provide:

-   -   (1) A method for manufacturing a high strength steel sheet,         comprising the steps of: heating a steel sheet containing at         least 0.10 mass % of carbon to either temperature in the         austenite single phase region or temperature in the         (austenite+ferrite) two-phase region; cooling the steel sheet to         cooling stop temperature as target temperature set within a         cooling temperature region ranging from Ms to (Ms−150° C.) to         allow a portion of non-transformed austenite to proceed to         martensitic transformation; and heating the sheet temperature to         temper the martensite, characterized in that the method further         comprises retaining the coldest part in the sheet widthwise         direction of the steel sheet at temperature in a temperature         range from the cooling stop temperature as the target         temperature to (the cooling stop temperature+15° C.) for a         period ranging from 15 seconds to 100 seconds (inclusive of 15         seconds and 100 seconds),         -   wherein “Ms” represents martensitic transformation start             temperature and the cooling temperature region is exclusive             of Ms and inclusive of (Ms−150° C.).     -   (2) The method for manufacturing a high strength steel sheet         of (1) above, further comprising subjecting the steel sheet to         hot dip galvanizing process or galvannealing process either:         between completion of the heating process to temperature in         either the austenite single phase region or the         (austenite+ferrite) two-phase region and completion of the         cooling process; or during the tempering process; or during a         process after the tempering process.     -   (3) The method for manufacturing a high strength steel sheet         of (1) or (2) above, wherein the steel sheet has a composition         including by mass %,         -   C: 0.10% to 0.73%,         -   Si: 3.0% or less,         -   Mn 0.5% to 3.0%,         -   P: 0.1% or less,         -   S: 0.07% or less,         -   Al: 3.0% or less,         -   N: 0.010% or less, and         -   remainder as Fe and incidental impurities.     -   (4) The method for manufacturing a high strength steel sheet         of (3) above, wherein the composition of the steel sheet further         includes by mass % at least one type of elements selected from         -   Cr: 0.05% to 5.0%,         -   V: 0.005% to 1.0%, and         -   Mo: 0.005% to 0.5%.     -   (5) The method for manufacturing a high strength steel sheet         of (3) or (4) above, wherein the composition of the steel sheet         further includes by mass % at least one type of elements         selected from         -   Ti: 0.01% to 0.1%, and         -   Nb: 0.01% to 0.1%.     -   (6) The method for manufacturing a high strength steel sheet of         any of (3) to (5) above, wherein the composition of the steel         sheet further includes, by mass %, B: 0.0003% to 0.0050%.     -   (7) The method for manufacturing a high strength steel sheet of         any of (3) to (6) above, wherein the composition of the steel         sheet further includes by mass % at least one type of elements         selected from         -   Ni: 0.05% to 2.0%, and         -   Cu: 0.05% to 2.0%.     -   (8) The method for manufacturing a high strength steel sheet of         any of (3) to (7) above, wherein the composition of the steel         sheet further includes by mass % at least one type of elements         selected from         -   Ca: 0.001% to 0.005%, and         -   REM: 0.001% to 0.005%.

It is thus possible to provide a high strength steel sheet being excellent in formability and exhibiting excellent stability in mechanical properties thereof. As a result, it is possible to reduce thickness of a steel sheet and weight thereof, thereby effectively reducing weight of an automobile body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)-1(c) are diagrams each showing a temperature pattern in a thermal treatment for forming martensite by a predetermined ratio by heating and rapidly cooling a steel sheet.

FIG. 2 is a diagram showing a temperature pattern in a thermal treatment in our method for manufacturing a high strength steel sheet.

DETAILED DESCRIPTION

Our steel sheets and methods will be described in detail hereinafter.

First, a steel sheet material as a starting steel material for manufacturing a high strength steel sheet is prepared by subjecting a steel sheet having a component composition adjusted to contain at least 0.10 mass % of carbon (“mass %” for a steel sheet component will be abbreviated to “%” hereinafter) to hot rolling process and, optionally, cold rolling process. These hot rolling and cold rolling processes are not particularly restricted and may be carried out according to the conventional methods.

The high strength steel sheet needs to contain at least 0.10% of carbon therein because carbon is an essential element in terms of increasing strength of the steel sheet, ensuring necessitated content of martensite and making austenite be retained at the room temperature.

Typical manufacturing conditions of a cold rolled steel sheet as a steel sheet material are as follows.

Manufacturing conditions of a cold rolled steel sheet include, for example: heating a steel material to temperature in the range of 1000° C. to 1300° C.; finishing hot rolling at temperature in the range of 870° C. to 950° C.; and subjecting a hot rolled steel sheet thus obtained to coiling at temperature in the range of 350° C. to 720° C., pickling, and cold rolling at rolling reduction rate in the range of 40% to 90% to obtain a cold rolled steel sheet (a steel sheet material).

It is acceptable in preparing a steel sheet material to skip at least a part of the hot rolling process by employing thin slab casting, strip casting or the like.

A high strength steel sheet is then manufactured from the (cold rolled) steel sheet material thus obtained according to our method including the following processes.

FIG. 2 shows one example of temperature pattern in thermal treatment of the method for manufacturing a high strength steel sheet.

A steel sheet material is heated for annealing to either temperature in the austenite single phase region or temperature in the (austenite+ferrite) two-phase region as shown in FIG. 2. The annealing temperature is not particularly restricted as long as it is equal to or higher than the temperature within the (austenite+ferrite) two-phase region. However, if the annealing temperature exceeding 1000° C. causes austenite grains to grow excessively, thereby coarsening gains of respective microstructures generated by cooling thereafter, which microstructures constitute a resulting steel sheet, to eventually deteriorate toughness and the like of the steel sheet. Accordingly, the annealing temperature is preferably 1000° C. or lower.

When the annealing time is shorter than 15 seconds, carbides already existing in a steel sheet prior to the annealing may not be dissolved sufficiently and/or reverse transformation of the microstructures of the steel sheet into austenite may not proceed sufficiently. When the annealing time exceeds 600 seconds, the processing cost increases due to too much energy consumption. Accordingly, the annealing time is to be in the range of 15 seconds to 600 seconds.

The steel sheet thus annealed is cooled to a first temperature region ranging from (martensite start temperature Ms−150° C.) to Ms (inclusive of (Ms−150° C.) and exclusive of Ms) as shown in FIG. 2. Cooling stop temperature: T1 (which will be referred to as “T1” hereinafter) as the target temperature is set within the first temperature region.

The purpose of this cooling process is to cool the steel sheet below the Ms point such that a portion of austenite proceeds to martensitic transformation. In a case where the lower limit of the first temperature region is set to be below (Ms−150° C.), most of non-transformed austenite proceeds to martensitic transformation by the cooling process and thus it is not possible to utilize microstructures like retained austenite which are effective in terms of improving formability of a steel sheet.

In a case where the upper limit of the first temperature region is set to exceed the Ms point, martensite may not have been formed in a sufficient content in the steel sheet when the cooling process is stopped, whereby tempered martensite cannot be reliably obtained in a sufficient content in the heating or tempering process thereafter. Accordingly, the first temperature region, within which T1 is set, is to range from (Ms−150° C.) to Ms, wherein (Ms−150° C.) is inclusive and Ms is exclusive.

The average cooling rate of a steel sheet until the temperature of the steel sheet drops to the first temperature region is not particularly restricted. However, the cooling rate lower than 3° C./s (“° C./s” represents “° C./second”) results in excess formation and growth of polygonal ferrite and precipitation of pearlite and the like, which makes it impossible to obtain the desired microstructure of a steel sheet. Accordingly, the average cooling rate from the annealing temperature to the first temperature region is to be at least 3° C./s.

It is particularly important that, when a portion of non-transformed austenite is made to proceed to martensitic transformation by cooling, the temperature of the coldest part in the sheet widthwise direction of a steel sheet is retained within the first temperature region (indicated as a hatched area in FIG. 2) and also in a temperature range or sub-region ranging from the target cooling stop temperature T1 to (T1+15° C.). In a case where the temperature of the coldest part of the steel sheet is below T1° C., non-transformed austenite proceeds to martensitic transformation excessively in some parts of the steel sheet to form too much martensite, thereby exceeding the target content thereof expected at the target temperature T1. As a result, variations in martensite cannot be eliminated and the desired properties cannot be stably obtained in these parts of the steel sheet even after retaining the steel sheet for a predetermined period. In a case where the temperature of the coldest part of the steel sheet exceeds (T1+15° C.), martensite is not formed sufficiently and fails to meet the target content thereof expected at the target temperature T1 in some part of the steel sheet. As a result, there arise variations in contents of bainite, retained austenite and tempered martensite formed during the heating or tempering process thereafter, whereby the desired properties cannot be stably obtained in the resulting steel sheet.

It is necessary to retain the coldest part of a steel sheet at a temperature in the range of T1 to (T1+15° C.) for a period ranging from 15 seconds to 100 seconds. Sheet temperature of parts other than the coldest part of a steel sheet may not sufficiently drop and these parts may fail to have the desired steel sheet microstructure, thereby generating variations in formability within the steel sheet, in a case where the retention time of the coldest part of the steel sheet at a temperature in the range of T1 to (T1+15° C.) is shorter than 15 seconds. A retention time exceeding 100 seconds would simply meaninglessly prolong the processing time because then an effect of making sheet temperature of parts other than the coldest part of a steel sheet follow the temperature of the coldest part, caused by the retention time, reaches a plateau.

“The coldest part” of a steel sheet represents the part at which sheet temperature is coldest in the sheet widthwise direction of the steel sheet. The coldest part of a steel sheet is normally an edge portion of the steel sheet, but may be another portion depending on the characteristics of a production line. In a case where there is a possibility that a portion other than an edge portion of a steel sheet will be the coldest part of the steel sheet, it is preferable that the steel sheet is in advance tested to investigate the coldest part thereof so that sheet temperature of the coldest part can be reliably controlled during the actual manufacturing process.

Manufacturing facilities are preferably equipped with a thermometer capable of confirming sheet temperature distribution across the entire sheet width of a steel sheet in terms of achieving reliable measurement of actual temperature of the coldest part of the steel sheet. If manufacturing facilities lack such a thermometer as described above, these facilities can still control thermal processing conditions by finding out the coldest part of a steel sheet by an experiment in advance as described above and measuring and controlling the temperature of the coldest part of the steel sheet thus determined.

Further, sectioning a steel sheet in the sheet widthwise direction into several blocks and carrying out feedback control of respective sheet temperatures in the respective blocks are effective in terms of reliably keeping sheet temperature of the steel sheet which is being retained within the temperature range of T1 to (T1+15° C.).

As described above, it is possible to remarkably decrease variations in mechanical properties such as tensile strength within a high strength steel sheet by retaining the coldest part of the steel sheet at a predetermined temperature for a predetermined period.

The mechanism of such a decrease in variations as described above is not clear. We believe that: if the concentration of martensite formed within a steel sheet has varied because temperatures of some parts of the steel sheet dropped too low from the Ms point due to variations in sheet temperature in the sheet thickness direction and the widthwise direction with respect to the sheet-feeding direction, the magnitude of martensite formation within the steel sheet can be made stable by carrying out the aforementioned unique thermal processing; and as a result the magnitude of martensitic transformation across the entire steel sheet is made uniform and mechanical properties of the steel sheet are rendered stable across the entire steel sheet.

Next, the steel sheet thus retained at temperature in the first temperature region is heated by a conventional method and subjected to martensite-tempering process as shown in FIG. 2.

Although a temperature range for this tempering process is not particularly restricted, the tempering temperature is preferably equal to or higher than 200° C. in view of tempering efficiency of martensite. In a case where the cooling stop temperature is equal to or higher than 200° C., the heating process for tempering can be omitted by simply retaining a steel sheet at a temperature in the temperature range equal to or higher than 200° C. The tempering temperature is preferably equal to or lower than 570° C. because carbides are precipitated from non-transformed austenite and desired microstructures may not be obtained when the upper limit of the tempering temperature exceeds 570° C.

Retention time after raising the temperature of a steel sheet to the tempering temperature is not particularly restricted. However, a retention time shorter than 5 seconds may result in insufficient tempering of martensite, which makes it impossible to obtain the desired microstructures in a resulting steel sheet and possibly deteriorates formability of the steel sheet. A retention time exceeding 1000 seconds, for example, causes carbides to be precipitated from non-trans-formed austenite and stable retained austenite having relatively high carbon concentration cannot be obtained as the final microstructure of a resulting steel sheet, whereby the resulting steel sheet may not have at least one of desired strength and ductility. Accordingly, the retention time of retaining a steel sheet for tempering is preferably in the range of 5 seconds to 1000 seconds.

The retention temperature in the aforementioned thermal and tempering processes need not be constant and may vary within such a predetermined temperature range as described above. In other words, a variation in the retention temperature within the predetermined temperature range does not have an adverse effect. Similar tolerance is applied to the cooling rate and the cooling rate may vary to some extent. Further, the steel sheet may be subjected to the relevant thermal treatments in any facilities as long as the required thermal history is satisfied. Yet further, subjecting a surface of the steel sheet to temper-rolling for shape correction and/or a surface treatment such as electrolytic plating after the thermal treatment is included.

The method for manufacturing a high strength steel sheet may further include subjecting the steel sheet to hot dip galvanizing process or galvannealing process (galvannealing process is combination of hot dip galvanizing and alloying process thereafter). In the case of carrying out hot dip galvanizing process or galvannealing process during the martensite-tempering process in the tempering temperature range, the total retention time at temperature in the tempering temperature region, including processing time for the hot dip galvanizing process or the galvannealing process, is still within the range of 5 seconds to 1000 seconds.

The hot dip galvanizing process and the galvannealing process are preferably carried out in a continuous galvanizing line.

In the method for manufacturing a high strength steel sheet, it is acceptable to complete the method to the final thermal treatment to obtain a high strength steel sheet and then subject the high strength steel sheet to hot dip galvanizing process and galvannealing process later.

A method for subjecting the steel sheet to hot dip galvanizing process and a method for subjecting the steel sheet to galvannealing process are typically carried out as follows.

A steel sheet is immersed in a plating bath and then coating weight is adjusted by gas wiping or the like. Aluminum content dissolved in the plating bath is preferably in the range of 0.12 mass % and 0.22 mass % in hot dip galvanizing and in the range of 0.08 mass % and 0.18 mass % in galvannealing, respectively. Temperature of a plating bath may be in the range of 450° C. to 500° C. in hot dip galvanizing. In a case where galvannealing is further carried out, temperature during the alloying process is preferably 570° C. or lower.

An alloying temperature exceeding 570° C. results in precipitation of carbides from non-transformed austenite and possibly formation of pearlite, which may lead to failure in obtaining at least one of good strength and good formability, as well as deterioration of anti-powdering property of a coating layer in a resulting coated steel sheet. However, the galvannealing process may not proceed smoothly when the alloying temperature is below 450° C. Accordingly, the alloying temperature is preferably equal to or higher than 450° C.

The coating weight per one surface of a steel sheet is preferably in the range of 20 g/m² to 150 g/m² in a case where the steel sheet is subjected to coating such as galvanizing. A coating weight less than 20 g/m² results in poor corrosion resistance, while a corrosion resisting effect reaches a plateau and production cost meaninglessly increases when the coating weight exceeds 150 g/m².

The alloy degree of a coating layer (i.e., Fe % or Fe content in a coating layer) is preferably in the range of 7% to 15%. An alloy degree of a coating layer less than 7% results in uneven alloying to deteriorate appearance quality of a resulting coated steel sheet and/or formation of what is called ζ phase in the coating layer to deteriorate sliding properties of a resulting coated steel sheet. An alloy degree of a coating layer exceeding 15 mass % results in excess formation of hard and brittle Γ phase to deteriorate coating adhesion properties of a resulting coated steel sheet.

In addition to the foregoing descriptions of the primary features regarding conditions in manufacturing a high strength steel sheet, a component composition of a steel sheet preferable as a steel sheet material for the manufacturing method will be described next.

C: 0.10% to 0.73%

At least 0.10% of carbon is required in the steel sheet as described above.

However, a carbon content exceeding 0.73% significantly hardens a welded portion and surrounding portions affected by welding heat, thereby deteriorating weldability of a resulting steel sheet. Accordingly, the upper limit of carbon content in steel is preferably 0.73%. Carbon content in steel is more preferably in the range of 0.15% to 0.48% (exclusive of 0.15% and inclusive of 0.48%).

Si: 3.0% or less

Silicon is a useful element which contributes to increasing strength of a steel sheet through solute strengthening. However, silicon content in steel exceeding 3.0% deteriorates: formability and toughness due to increase in content of solute Si in polygonal ferrite and bainitic ferrite; and coatability and coating adhesion of plating when the steel sheet is subjected to hot dip galvanizing. Accordingly, Si content in steel is 3.0% or less, preferably 2.6% or less, and more preferably 2.2% or less.

The silicon content in steel is preferably at least 0.5% because silicon is a useful element in terms of suppressing formation of carbide and facilitating formation of retained austenite. However, silicon need not be added and thus Si content may be zero % in a case where formation of carbide is suppressed by only aluminum.

Mn: 0.5% to 3.0%

Manganese is an element which effectively increases steel strength. A manganese content less than 0.5% in steel causes carbide to be precipitated at temperature higher than the temperature at which bainite and martensite are formed when a steel sheet is cooled after annealing, thereby making it impossible to reliably obtain a sufficient content of hard phase contributing to steel strengthening. An Mn content exceeding 3.0% may deteriorate forgeability of the steel. Accordingly, the Mn content in the steel is preferably in the range of 0.5% to 3.0% and more preferably in the range of 1.5% to 2.5%.

P: 0.1% or less

Phosphorus is a useful element in terms of increasing steel strength. However, a phosphorus content in steel exceeding 0.1%; makes steel brittle due to grain boundary segregation of phosphorus to deteriorate impact resistance of a resulting steel sheet; and significantly slows galvannealing (alloying) rate down in a case the steel sheet is subjected to galvannealing. Accordingly, the phosphorus content in the steel is 0.1% or less and preferably 0.05% or less.

The lower limit of the phosphorus content in the steel is preferably around 0.005% because an attempt to reduce the phosphorus content below 0.005% significantly increases production costs, although the phosphorus content in the steel is to be decreased as best as possible.

S: 0.07% or less

Sulfur forms inclusions such as MnS and may be a cause of deterioration of impact resistance and generation of cracks along metal flow at a welded portion of a steel sheet. It is thus preferable that sulfur content in the steel is reduced as best as possible. However, decreasing the sulfur content in the steel to an exorbitantly low level increases production costs. Accordingly, presence of sulfur in the steel is tolerated unless the sulfur content in the steel exceeds 0.07% or so. The sulfur content in the steel is preferably 0.05% or less, and more preferably 0.01% or less. The lower limit of the sulfur content in the steel is around 0.0005% in view of production costs because decreasing the sulfur content in the steel below 0.0005% significantly increases production costs.

Al: 3.0% or less

Aluminum is a useful element added as a deoxidizing agent in a steel manufacturing process. However, an aluminum content exceeding 3.0% may deteriorate ductility of a steel sheet due to too much inclusion in the steel sheet. Accordingly, the aluminum content in steel is 3.0% or less and preferably 2.0% or less.

Further, aluminum is a useful element in terms of suppressing formation of carbides and facilitating formation of retained austenite. The aluminum content in steel is preferably at least 0.001% and preferably at least 0.005% to sufficiently obtain this good effect of aluminum.

The aluminum content represents content of aluminum contained in a steel sheet after deoxidization.

N: 0.010% or less

Nitrogen is an element which most significantly deteriorates anti-aging property of steel and thus the content thereof in the steel is preferably decreased as best as possible. However, the presence of nitrogen in the steel is tolerated unless the nitrogen content in the steel exceeds 0.010% or so. The lower limit of the nitrogen content in the steel is around 0.001% in view of production costs because decreasing the nitrogen content in the steel below 0.001% significantly increases production costs.

The composition of the steel sheet may further include, in addition to the aforementioned optional components other than carbon, the following components in an appropriate manner.

At least one type of element selected from Cr: 0.05% to 5.0%, V: 0.005% to 1.0%, and Mo: 0.005% to 0.5%

Chromium, vanadium and molybdenum are elements which each suppress formation of pearlite when a steel sheet is cooled from the annealing temperature. These good effects of Cr, V and Mo are obtained when the contents of Cr, V and Mo in steel are at least 0.05%, at least 0.005% and at least 0.005%, respectively. However, the contents of Cr, V and Mo in steel exceeding 5.0%, 1.0% and 0.5%, respectively, results in too much formation of hard martensite, which strengthens a resulting steel sheet too much to make the steel sheet brittle. Accordingly, in a case where the composition of the steel sheet includes at least one of Cr, V and Mo, the contents thereof are Cr: 0.05% to 5.0%, V: 0.005% to 1.0%, and Mo: 0.005% to 0.5%.

At least one type of element selected from Ti: 0.01% to 0.1%, and Nb: 0.01% to 0.1%

Titanium and niobium are useful elements in terms of precipitate strengthening/hardening of steel. Titanium and niobium can each cause this effect when the contents thereof in steel are at least 0.01%, respectively. In a case where at least one of the Ti content and Nb content in the steel exceeds 0.1%, formability and shape fixability of a resulting steel sheet deteriorate. Accordingly, in a case where the steel sheet composition includes Ti and Nb, the contents thereof are Ti: 0.01% to 0.1%, and Nb: 0.01% to 0.1%, respectively.

B: 0.0003% to 0.0050%

Boron is a useful element in terms of suppressing formation and growth of ferrite from austenite grain boundary. This good effect of boron can be obtained when the boron content in the steel is at least 0.0003%. However, a boron content in the steel exceeding 0.0050% deteriorates formability of a resulting steel sheet. Accordingly, when the steel sheet composition includes boron, the boron content in steel is B: 0.0003% to 0.0050%.

At least one type of elements selected from Ni: 0.05% to 2.0%, and Cu: 0.05% to 2.0%

Nickel and copper are elements which each effectively increase strength of steel. Further, these elements each cause an effect of facilitating internal oxidation of a surface layer portion of a steel sheet to improve coating adhesion property in a case the steel sheet is subjected to galvanizing or galvannealing. These good effects of Ni and Cu are obtained when the contents thereof in the steel are at least 0.05%, respectively. In a case where at least one of Ni content and Cu content in the steel exceeds 2.0%, formability of a resulting steel sheet deteriorates. Accordingly, in a case where the steel sheet composition includes Ni and Cu, the contents thereof are Ni: 0.05% to 2.0%, and Cu: 0.05% to 2.0%, respectively.

At least one element selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%

Calcium and REM are useful elements in terms of making sulfides spherical to lessen the adverse effects of the sulfides on stretch flangeability of a steel sheet. Calcium and REM can each cause this effect when the contents thereof in steel are at least 0.001%, respectively. In a case where at least one of the Ca content and REM content in the steel exceeds 0.005%, inclusions increase to cause surface defects, internal defects and the like of a resulting steel sheet. Accordingly, in a case where the steel sheet composition includes Ca and REM, the contents thereof are Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%, respectively.

Components other than those described above are Fe and incidental impurities in the steel sheet. However, our steel sheets do not exclude the possibility that the steel composition thereof includes a component other than those described above unless inclusion of the component has an adverse effect.

EXAMPLES Example 1

Our steel sheets and methods will be described further in detail by Examples hereinafter. These Examples, however, do not restrict this disclosure by any means. Needless to say, any changes in structure can be made without having an adverse effect.

A steel material, obtained from steel having a component composition as shown in Table 1 by using ingot techniques, was heated to 1200° C. and subjected to finish hot rolling at 870° C. to obtain a hot rolled steel sheet. The hot rolled steel sheet was subjected to coiling at 650° C., pickling, and cold rolling at rolling reduction rate of 65% to obtain a cold rolled steel sheet having sheet thickness: 1.2 mm. The cold rolled steel sheet thus obtained was subjected to thermal treatment under the conditions shown in Table 2.

The thermal treatment temperatures (the annealing temperatures) shown in Table 2 were all within either the austenite single phase region or the (austenite+ferrite) two-phase region, except for that of sample No. 4.

Some of the cold rolled steel sheet samples were each subjected to hot dip galvanizing or galvannealing either during the tempering process or after the tempering process. The hot dip galvanizing process was carried out such that respective surfaces of a cold rolled steel sheet sample were coated at coating weight (per one surface): 50 g/m² at plating path temperature: 463° C. The galvannealing process was carried out such that respective surfaces of a cold rolled steel sheet sample were first subjected to coating at coating weight (per one surface): 50 g/m² at plating path temperature: 463° C. and then alloying, under alloying conditions adjusted as required, at temperature equal to or lower than 550° C. to achieve alloy degree (i.e., Fe % or Fe content in a coating layer) of 9 mass %.

Each of the steel sheet samples thus obtained was subjected to temper-rolling at rolling reduction rate (elongation rate): 0.3% either directly after the thermal treatment in a case where the sample was not subjected to any coating process or after the hot dip galvanizing process or the galvannealing process in a case where the sample was subjected to a coating process.

TABLE 1 Steel Steel sheet component (mass %) type C Si Mn Al P S N Cr V Mo Ti Nb B Ni Cu Ca REM Note A 0.400 1.99 1.98 0.036 0.012 0.0040 0.0023 — — — — — — — — — — Steel B 0.310 2.02 1.52 0.040 0.010 0.0030 0.0041 — — — — — — — — — — Steel C 0.090 0.80 2.50 0.042 0.015 0.0050 0.0040 — — — — — — — — — — Comparative steel D 0.302 2.01 2.03 0.039 0.009 0.0040 0.0037 — — — — — — — — — — Steel E 0.402 1.80 0.50 0.041 0.010 0.0040 0.0037 0.9 — — — — — — — — — Steel F 0.498 2.05 1.50 0.039 0.013 0.0040 0.0032 — 0.05 — — — — — — — — Steel G 0.604 1.98 1.49 0.041 0.010 0.0030 0.0039 — — — — — — — — — — Steel H 0.298 2.00 1.81 0.037 0.029 0.0030 0.0041 — — 0.03 — — — — — — — Steel I 0.301 2.41 1.92 0.037 0.029 0.0030 0.0041 — — — — 0.03 — — — — — Steel J 0.412 1.10 1.52 1.02 0.013 0.0030 0.0037 — — — — — — 0.20 0.20 — — Steel K 0.480 1.70 1.30 0.038 0.012 0.0030 0.0041 — — — 0.020 — 0.0015 — — — — Steel L 0.185 1.52 2.33 0.041 0.011 0.0040 0.0029 — — — — — — — — 0.002 — Steel M 0.145 1.51 2.09 0.039 0.013 0.0030 0.0040 — — — — — — — — — 0.003 Steel

TABLE 2 Average Cooling rate down Target to first cooling stop Retention Temperature Annealing process temperature temperature: time of the range of the Ms − Tempering process Sample Steel Temperature Time region T1 coldest part coldest part Ms 150° C. Temperature Time No. type (° C.) (s) (° C./s) (° C.) (s) (° C.) (° C.) (° C.) (° C.) (s) Note 1 A 900 300 18 250 18 250~260 325 175 400 120 Example 2 B 900 200 20 400 30 393~400 378 228 420 100 Comparative Example 3 B 890 150 50 100 20 102~110 378 228 400 90 Comparative Example 4 B 670 200 15 250 20 252~260 378 228 400 120 Comparative Example 5 B 900 180 20 270 20 270~273 378 228 400 90 Example 6 B 900 180 15 240 25 210~245 378 228 400 90 Comparative Example 7 C 900 180 20 290 20 290~294 443 293 360 90 Comparative Example 8 D 890 200 15 250 18 252~258 366 216 390 90 Example 9 E 880 300 20 250 20 251~259 358 208 410 120 Example 10 F 870 400 15 190 20 191~201 297 147 400 300 Example 11 G 870 500 18 170 30 171~182 252 102 420 400 Example 12 H 900 200 20 225 20 226~232 364 214 400 180 Example 13 I 900 200 20 220 20 220~233 348 198 410 350 Example 14 J 900 200 20 220 20 221~227 318 168 400 300 Example 15 K 880 300 15 165 18 167~174 299 149 400 400 Example 16 L 900 200 35 280 16 282~285 407 257 390 90 Example 17 M 900 200 20 290 20 290~299 412 262 400 100 Example

Various properties of each of the steel sheet samples and the coated steel sheet samples thus obtained were evaluated by the following methods.

A tensile test was carried out according to JIS Z 2241 by using a JIS No. 5 test piece collected from the steel sheet sample in a direction orthogonal to the rolling direction thereof. TS (tensile strength) and T.EL (total elongation) of the test piece were measured and the product of the tensile strength and the total elongation (TS×T. EL) was calculated to evaluate balance between strength and formability (ductility) of the steel sheet sample. TS×T. EL≧20000 (MPa·%) is evaluated to be good balance between strength and elongation.

Stretch flangeability of each of the steel sheet samples and the coated steel sheet samples thus obtained was evaluated according to The Japan Iron and Steel Federation Standard (JFS) T1001 by: cutting the steel sheet sample into a test piece (100 mm×100 mm); forming a hole (diameter: 10 mm) by punching in the test piece with clearance corresponding to 12% of the sheet thickness between a steel sheet edge and the hole; pushing a 60° cone punch into the hole in a state where the test piece was set on a die (inner diameter: 75 mm) with fold pressure: 88.2 kN exerted thereon; measuring a critical hole diameter at crack initiation; and calculating a critical hole expansion ratio λ (%) according to Formula (1) below:

Critical hole expansion ratio λ (%)={(D _(f) −D ₀)/D ₀}×100  (1).

In Formula (1), D_(f) represents critical hole diameter at crack initiation (mm) and D_(o) represents the initial hole diameter (mm).

Further, balance between strength and stretch-flangeability of the steel sheet sample was evaluated by calculating the product of strength and critical hole expansion ratio (TS×λ) by using λ thus determined through measurement.

Stretch-flangeability is evaluated to be good when TS×λ≧25000 (MPa·%).

The results obtained by the measurements described above are shown in Table 3.

TABLE 3 TS × TS × T.EL λ Sample Steel TS T.EL λ (MPa · (MPa · No. type (MPa) (%) (%) %) %) Note 1 A 1477 22 18 32494 26586 Example 2 B 1212 20 17 24240 20604 Compara- tive Example 3 B 1520 11 46 16720 69920 Compara- tive Example 4 B  836 22 40 18392 33440 Compara- tive Example 5 B 1382 16 44 22112 60808 Example 6 B 1451 13 44 18863 63844 Compara- tive Example 7 C 1119 8 50  8952 55950 Compara- tive Example 8 D 1370 16 37 21920 50690 Example 9 E 1471 19 30 27949 44130 Example 10 F 1563 18 17 28134 26571 Example 11 G 1678 19 15 31882 25170 Example 12 H 1482 14 35 20748 51870 Example 13 I 1474 18 41 26532 60434 Example 14 J 1498 16 35 23968 52430 Example 15 K 1750 12 18 21000 31500 Example 16 L 1198 20 29 23960 34742 Example 17 M  992 25 40 24800 39680 Example

As is obvious from Table 3, the steel sheet samples manufactured according to our method all satisfied tensile strength of at least 980 MPa, (TS×T. EL)≧20000 (MPa·%) and (TS×λ)≧25000 (MPa·%). That is, it is confirmed from Table 3 that each of the steel sheet samples has satisfactorily high strength and excellent formability in particular excellent stretch-flangeability.

In contrast, sample No. 4, in which the annealing temperature failed to reach the (austenite+ferrite) two-phase region, did not obtain the desired microstructures of a steel sheet and had tensile strength (TS) below 980 MPa and (TS×T. EL) below 20000 (MPa·%), although (TS×λ)≧25000 (MPa·%) and stretch-flangeability was relatively good therein.

Each of sample No. 2 and sample No. 3, in which T1 was beyond the first temperature region, did not obtain the desired microstructures of a steel sheet and failed to satisfy at least one of (TS×T. EL)≧20000 (MPa·%) and (TS×λ)≧25000 (MPa·%), although it met tensile strength (TS)≧980 MPa.

Sample No. 6, in which temperature of the coldest part of the steel sheet dropped below the target temperature during the retention time, i.e., was outside our range, did not obtain the desired microstructures of a steel sheet and failed to satisfy (TS×T. EL)≧20000 (MPa·%), although it met tensile strength (TS)≧980 MPa.

Sample No. 7, of which carbon content was outside our range, did not obtain the desired microstructures of a steel sheet and failed to have the desired properties of the steel sheet.

Example 2

Further, samples Nos. 18-22 prepared by using steel type A shown in Table 1 were subjected to thermal treatment conditions show in Table 4, respectively. Table 5 shows results of investigating mechanical properties and variations therein for each of these samples. Variations in mechanical properties of each steel sheet sample were determined by: cutting 20 sheets of test materials (length in the rolling direction: 40 mm×width: 250 mm) from a portion (length of the rolling direction: 1000 mm) of the steel sheet sample, wherein these test materials to be evaluated were originally evenly distributed (located) across the entire width of the steel sheet (i.e., from one edge via the center portion to the other edge of the steel sheet) and then cut and collected, respectively; obtaining JIS No. 5 test pieces from these 20 test materials, respectively; subjecting each of the respective JIS No. 5 test pieces to tensile test; and calculating standard deviations of tensile strength and T. EL. for each of the test pieces. Standard deviation a of tensile strength ≦10 MPa and standard deviation σ of T. EL.≦2.0% are evaluated to be good, respectively.

TABLE 4 Average cooling Target Retention rate down cooling time of to first stop the Temperature Sam- Annealing process temperature temperature: coldest range of the Tempering process ple Steel Temperature Time region T1 part coldest part Ms Ms − 150° C. Temperature Time No. type (° C.) (s) (° C./s) (° C.) (s) (° C.) (° C.) (° C.) (° C.) (s) Note 18 A 900 250 20 250 18 251~253 325 175 400 120 Example 19 A 900 250 20 250 20 245~263 325 175 400 100 Comparative Example 20 A 900 300 20 300  2 300~302 325 175 400 100 Comparative Example 21 A 900 300 20 300  7 300~305 325 175 400 120 Comparative Example 22 A 900 300 20 300 30 300~308 325 175 400 120 Example

TABLE 5 Standard Standard Sample deviation σ deviation σ No. of TS (MPa) of T. EL. (%) Note 18  5 0.9 Example 19 12 1.9 Comparative Example 20 22 3.1 Comparative Example 21 15 2.5 Comparative Example 22  6 1.3 Example

As shown in Table 5, sample No. 18 and sample No. 22 subjected to our thermal treatment each satisfy standard deviation σ of tensile strength ≦10 MPa and standard deviation a of T. EL.≦2.0%, i.e., good stability in mechanical properties. In contrast, sample No. 19 having temperature of the coldest part of the steel sheet beyond the range of T1 to (T1+15° C.) and samples Nos. 20 and 21 each having retention time of the coldest part of the steel sheet beyond the range of 15 s to 1000 s all exhibit large variations, i.e., at least one of standard deviation σ of tensile strength >10 MPa and standard deviation σ of T. EL.>2.0%.

Further, the mechanical properties and variations therein were analyzed for each of our steel sheet samples shown in Table 3 in the same manner as described above in connection with samples Nos. 18-22. It was confirmed that our steel sheet samples each satisfied both standard deviation σ of tensile strength 10 MPa and standard deviation σ of T. EL.≦2.0%, i.e., good mechanical stability.

INDUSTRIAL APPLICABILITY

Our high strength steel sheet, being excellent in formability and tensile strength (TS) and exhibiting good stability in mechanical properties, is very useful in the industrial fields of automobile, electric appliances and the like and in particular contributes to reducing weight of automobile body. 

1. A method for manufacturing a high strength steel sheet comprising: heating a steel sheet containing at least 0.10 mass % of carbon to either a temperature in an austenite single phase region or a temperature in an (austenite+ferrite) two-phase region; cooling the steel sheet to a cooling stop temperature as a target temperature set within a cooling temperature region ranging from Ms to (Ms−150° C.) to allow a portion of non-transformed austenite to proceed to martensitic transformation; retaining a coldest part in a sheet widthwise direction of the steel sheet at a temperature in a temperature range from the cooling stop temperature as the target temperature to (the cooling stop temperature+15° C.) for 15 seconds to 100 seconds; and heating the sheet to a temperature to temper said martensite, wherein “Ms” represents martensitic transformation start temperature and said cooling temperature region is exclusive of Ms and inclusive of (Ms−150° C.).
 2. The method of claim 1, further comprising subjecting the steel sheet to a hot dip galvanizing process or a galvannealing process either: between completion of the heating process to a temperature in either the austenite single phase region or the (austenite+ferrite) two-phase region and completion of the cooling process; or during the tempering process; or during a process after the tempering process.
 3. The method of claim 1, wherein the steel sheet has a composition including by mass %, C: 0.10% to 0.73%, Si: 3.0% or less, Mn 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, N: 0.010% or less, and remainder as Fe and incidental impurities.
 4. The method of claim 3, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Cr: 0.05% to 5.0%, V: 0.005% to 1.0% and Mo: 0.005% to 0.5%.
 5. The method of claim 3, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ti: 0.01% to 0.1% and Nb: 0.01% to 0.1%.
 6. The method of claim 3, wherein the composition of the steel sheet further comprises, by mass %, B: 0.0003% to 0.0050%.
 7. The method of claim 3, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to 2.0%.
 8. The method of claim 3, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.
 9. The method of claim 2, wherein the steel sheet has a composition including by mass %, C: 0.10% to 0.73%, Si: 3.0% or less, Mn 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, N: 0.010% or less, and remainder as Fe and incidental impurities.
 10. The method of claim 4, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ti: 0.01% to 0.1% and Nb: 0.01% to 0.1%.
 11. The method of claim 4, wherein the composition of the steel sheet further comprises, by mass %, B: 0.0003% to 0.0050%.
 12. The method of claim 5, wherein the composition of the steel sheet further comprises, by mass %, B: 0.0003% to 0.0050%.
 13. The method of claim 4, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to 2.0%.
 14. The method of claim 5, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to 2.0%.
 15. The method of claim 6, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to 2.0%.
 16. The method of claim 4, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.
 17. The method of claim 5, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.
 18. The method of claim 6, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.
 19. The method of claim 7, wherein the composition of the steel sheet further comprises by mass % at least one element selected from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%. 